1. Field of the Invention
The present invention relates to an image reproducing technique for processing a high dynamic range input signal to reproduce an appropriate image signal suitable for an output or display device, which technique is applied to image reproducing and displaying apparatuses including liquid crystal displays or television monitors used along with digital cameras or video cameras.
2. Description of Related Art
Many studies and proposals have been made of image reproducing apparatus, such as liquid crystal monitors, used in or together with photographing apparatuses or image recording apparatuses (e.g., digital cameras or video cameras). For example, JP 2002-305684A discloses an electronic camera used in an image recording system in which multiple images of a same object taken under different exposure conditions are combined to generate and record a high dynamic range image at a tone resolution (or a gradation width) designated by a user. In this publication, the electronic camera performs high dynamic range photographing and reproduces appropriate image data suitable for an output device by selecting a tone resolution level lower than the that of the recorded image when displaying or outputting the photographed dynamic range image on a liquid crystal display or from a printer.
Another publication JP 11-317930A discloses a digital camera that allows a user or an operator to recognize a thumbnail image at which page he/she is browsing. With this publication, the operator can set the number of thumbnail images to be displayed at a page when displaying thumbnail images on a display screen, and display of page data is controlled by control means.
Other than these examples, there are many methods for displaying images so as to be easily viewable by the user in an image reproducing and display apparatus used along with a digital camera or other photographing apparatus.
However, with the former publication (JP 2002-305684), when actually displaying or outputting the composite image directly on a monitor screen or from a printer, an 8-bit or 6-bit synthetic image is generated in accordance with the display resolution of the output apparatus. Accordingly, if the output apparatus does not have a sufficient dynamic range, delicate nuance of the tone is lost. The gradation or the tone can be maintained if a high-performance output apparatus with a sufficient dynamic range is used; however, it imposes increased cost and expense.
Another problem arises when a portion of a high dynamic range image is zoomed or scrolled on the display screen. If dynamic range correction is performed on the zoomed or scrolled image using only a part of high dynamic range image data residing in the display frame area, and if a high luminance object comes into the frame, then the relative luminance level of a substantially white object is reduced. As a result, the original highlight image becomes dull.
For instance, when high dynamic range photographing is carried out within the frame shown in FIG. 3B, the exposure level is changed between the shadow in which dark objects are mainly photographed and the highlight in which bright objects are mainly photographed (by splitting the incident light so as to allow the beams to strike two sensors or CCD arrays, or alternatively, by photographing a still object twice at different exposure levels). In general, the exposure level of a highlight region is selected so as to prevent highlight information from being lost. If the scene shown in FIG. 3B is taken at an exposure level selected so as to take advantage of the dynamic range of the photographic apparatus, the luminance distribution shown in FIG. 1A is obtained. In this case, the snowcap of the mountain is the most highlighted area. (Since a physical quantity “absolute luminance” cannot be measured with a CCD camera, the saturation level of the imaging element becomes highlight.)
Next, if the scene shown in FIG. 3A including a high luminous object, such as the sun, is taken or displayed, the luminance distribution becomes one illustrated in FIG. 1B because the exposure level is selected so as not to be saturated at the highest luminance portion (that is, the sun).
In comparison with the distribution of FIG. 1A, the luminance level of the snowcap, which is the highest in FIG. 1A, slightly shifts to the gray side because it is lower than that of the luminous object such as the sun. As a result, snow white in FIG. 3B slightly turns into gray in FIG. 3A, and the image is dulled.
Furthermore, if dynamic range correction is made using the entire image data to perform tone compression down to 8-bit or 6-bit gradation, the subtle nuance of the image is lost and the user cannot see the delicate tone of the high dynamic range image taken by the camera.
If the difference in luminance level between the contour line of the major object and the background is very small (for example, with a level difference of 1 or 2 in 16-bit digital value), the boundary between the main object and the background becomes unclear in the reproduced image, as illustrated in FIG. 2A and FIG. 2B. In addition, for those portions with continuous gradation with a smooth tone change, pseudo contour line (which appears due to a quantization problem although no such contour line actually exists) is likely to be produced.